Breathing strategies have been considered an important component of pulmonary rehabilitation and refer to a range of techniques, including/which includes the diaphragmatic breathing (DB). The principal aim of DB is to improve the participation of the abdominal motion while reducing the accessory muscles activity. The role of DB as an adjunctive treatment modality in the rehabilitation of COPD patients remains controversial. Despite the conflicting results, improvement of the abdominal motion and a reduction of thoracic excursion, during voluntarily DB, has been described as a common finding of several studies. The investigators hypothesized that a 4-week DBTP could induce higher participation of the diaphragm and this modification would relieve respiratory symptoms and improve exercise tolerance and the HRQoL. Therefore, in this randomized controlled trial the investigators aimed to test the effects of a short-term DBTP on thoracoabdominal motion, pulmonary function, diaphragmatic mobility, dyspnea, HRQoL and exercise tolerance in patients with mild to severe COPD.